The Prophet ﷺ said: (The imam is appointed only to be followed) and the Imam should follow the example set by the prophet ﷺ. The person following the imam should respect him and regard him in a way that befits him, because the Imam stands in the position of the Prophet ﷺ so that he may establish Allah’s law, and the Muslims should follow what the Prophet ﷺ was upon.
The maliki’s have said that it is disliked for the Imam to pray nafl in the mihrab, because he should only stand there when he is Imam. If he stands there and prays, others may think he is praying an obligatory prayer and join him. They said it is disliked for the Imam to sit in the mihrab after the prayer the way a person who is praying sits, and it is possible for him to move out of this dislike by changing the way he sits, due to the hadith of Samura bin Jundub that “when the Prophet ﷺ had prayed a prayer he turned and faced us”.
But if the Imam does not delude others into thinking that he is performing an obligatory prayer, and whoever behind him knows that he is performing a voluntary prayer, then there is no harm in him praying there.
It would be nice if muslims were committed to observing the sanctity of the houses of Allah SWA, and we stayed away from temptations and disagreements within the mosque; because the mosque is built for worship and obedience to Allah SWA, Allah SWA says: (None should inhabit the mosques of Allah except those who believe in Allah and the Last Day, establish their prayers and pay the obligatory charity, and fear none except Allah. May these be among the guided.)